An electrophotographic photoreceptor may have various structures depending on the characteristics required or electrophotographic processes to be employed.
A system in which a photoreceptor comprises a support having thereon at least one photoconductive layer and, if necessary, an insulating layer on the surface thereof is widely employed. The photoreceptor comprising a support and at least one photoconductive layer is subjected to ordinary electrophotographic processing for image formation including charging, imagewise exposure, development and, if desired, transfer.
Electrophotographic photoreceptors have also been used widely as offset printing plate precursor for direct printing plate making. In particular, a direct electrophotographic lithographic printing system has recently been acquiring a greater importance as a system providing hundreds to thousands of prints of high image quality.
Binders which are used in the photoconductive layer should themselves have film-forming properties and the capability of dispersing photoconductive particles therein. Also, when formulated into a photoconductive layer, the binders should have satisfactory adhesion to a support. They also must have various electrostatic characteristics and image-forming properties, such that the photoconductive layer exhibits excellent electrostatic capacity, small dark decay and large light decay, hardly undergo fatigue before exposure, and maintain these characteristics in a stable manner against change of humidity at the time of image formation.
Binder resins which have been conventionally used include silicone resins (see JP-B-34-6670, the term "JP-B" as used herein means an "examined published Japanese patent application"), styrene-butadiene resins (see JP-B-35-1960), alkyd resins, maleic acid resins and polyamides (see Japanese JP-B-35-11219), vinyl acetate resins (see JP-B-41-2425), vinyl acetate copolymer resins (see JP-B-41-2426), acrylic resins (see JP-B-35-11216), acrylic ester copolymer resins (see JP-B-35-12129, JP-B-36-81510, and JP-B-4l-l3946), etc. However, electrophotographic photosensitive materials using these known resins have a number of disadvantages, i.e., poor affinity for photoconductive particles (poor dispersion of a photoconductive coating composition); low photoconductive layer charging properties; poor reproduced image quality, particularly dot reproducibility or resolving power; susceptibility of the reproduced image quality to influences from the environment at the time of electrophotographic image formation, such as high temperature and high humidity conditions or low temperature and low humidity conditions; and the like.
To improve the electrostatic characteristics of a photoconductive layer, various approaches have hitherto been taken. For example, incorporation of a compound containing an aromatic ring or furan ring containing a carboxyl group or a nitro group either alone or in combination with a dicarboxylic acid anhydride into a photoconductive layer as disclosed in JP-B-42-6878 and JP-B-45-3073 has been proposed. However, the thus improved photosensitive materials still have insufficient electrostatic characteristics, particularly light decay characteristics. The insufficient sensitivity of these photosensitive materials has been compensated for by incorporation a large quantity of a sensitizing dye into the photoconductive layer. However, photosensitive materials containing a large quantity of a sensitizing dye undergo considerable whiteness deterioration, which means reduced quality as a recording medium, sometimes causing a deterioration in dark decay characteristics, resulting in a failure to obtain a satisfactory reproduced image.
On the other hand, JP-A-60-10254 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") suggests control of the average molecular weight of a resin to be used as a binder of the photoconductive layer. According to this suggestion, the combined use of an acrylic resin having an acid value of from 4 to 50 whose average molecular weight is distributed within two ranges, i.e., a range of from 1.times.10.sup.3 to 1.times.10.sup.4 and a range of from 1.times.10.sup.4 and 2.times.10.sup.5, would improve the electrostatic characteristics, particularly reproducibility, as a PPC photoreceptor on repeated use, moisture resistance and the like.
In the field of lithographic printing plate precursors, extensive studies have been conducted to provide binder resins for a photoconductive layer having electrostatic characteristics compatible with printing characteristics. Examples of binder resins so far reported to be effective for oil-desensitization of a photoconductive layer include a resin having a molecular weight of from 1.8.times.10.sup.4 to 10.times.10.sup.4 and a glass transition point of from 10.degree. C. to 80.degree. C. obtained by copolymerizing a (meth)acrylate monomer and a copolymerizable monomer in the presence of fumaric acid in combination with a copolymer of a (meth)acrylate monomer and a copolymerizable monomer other than fumaric acid as disclosed in JP-B-50-31011; a terpolymer containing a (meth)acrylic ester unit with a substituent having a carboxyl group at least 7 atoms distant from the ester linkage as disclosed in JP-A-53-54027; a tetra- or pentapolymer containing an acrylic acid unit and a hydroxyethyl (meth)acrylate unit as disclosed in JP-A-54-20735 and JP-A-57-202544; a terpolymer containing a (meth)acrylic ester unit with an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxyl group as disclosed in JP-A-58-68046; and the like.
However, none of these resins proposed has proved to be satisfactory for practical use in charging properties, dark charge retention, photosensitivity, and surface smoothness of the photoconductive layer.
The binder resins proposed for use in electrophotographic lithographic printing plate precursors were also proved by actual evaluations to give rise to problems relating to electrostatic characteristics, background staining of prints, and moisture resistance.
Electrophotographic recording systems utilizing a laser beam as a light source have recently been developed. In this system, laser light emitted from a laser and condensed through an f.theta. lens is reflected on a polygon mirror to form a scan image on a photoreceptor, and the image is then developed and, if necessary, transferred.
With the recent development of semi-conductor lasers of low output, e.g., of from about 4 mW to 25 mW, development of a photosensitive material having a sensitivity in the wavelength region of 700 nm or more is required. An electrophotographic photoreceptor applicable to processing using such a low output laser must possess special characteristics different form those required for conventional electrophotographic photoreceptors. Particularly important is the property that the photoreceptor should exhibit sufficient sensitivity to the near infrared to infrared light as well satisfactory dark charge retention.
The combination of a photoconductive substance-binder resin dispersed system with various kinds of near infrared to infrared spectral sensitizing dyes to form an electrophotographic photoreceptor as disclosed, e.g., in JP-A-58-58554, JP-A-58-42055, JP-A-58-59453, and JP-A-57-46245 is known. These photoreceptors, however, have turned out to have insufficient dark charge retention and photosensitivity. As stated above, in using a laser, e.g., a semi-conductor laser, as a light source, exposure of a photoconductive layer is effected by scanning so that the time form charging through the end of exposure becomes longer than that required in the conventional exposure to visible light over the entire surface thereof. The charge on the unexposed area should be sufficiently retained over that time. Thus, dark charge retention is one of the extremely important characteristics required for electrophotographic photoreceptors to be used in scanning exposure. The above-described conventional photoreceptors have been unsatisfactory in this point.
Taking the low output of the light source into consideration, sufficiently high sensitivity in the near infrared to infrared region is an important characteristic as well. Conventional photoreceptors are also unsatisfactory in this respect.